The present invention relates to a heat-shrinkable laminate film showing an adequate rigidity and a favorable stretchability, which comprises outer layers of a polyolefin, a gas-barrier layer of a copolymer of vinylidene chloride (hereinafter referred to as VDC), at least one intermediate layer of an ionomer and adhesive layers disposed between any of the above layers, the total thickness of the intermediate layer(s) of an ionomer being 5 to 20% of the total thickness of the heat-shrinkable laminate film.
For packaging foodstuffs of non-uniform and irregular shapes, a method of heat-shrink packaging (also called the skin-packaging) while using a heat-shrinkable packaging material is generally applied, and since it is demanded to preserve the thus packaged foodstuff sanitarily for a long time period, it is necessary that the packaging material used for that purpose is impermeable to gases, particularly to gaseous oxygen, in other words, the packaging material must have gas-barrier property.
As the heat-shrinkable gas-barrier material, as has been well known, a single film of a VDC has been utilized.
However, because of the poor mechanical properties of the single film of a VDC at lower temperatures, a laminate film comprising outer layers of a polyolefin and a gas-barrier layer of a VDC has been developed. For instance, Japanese Patent Application Laying-Open No. 58-128821 (1983) discloses a laminate film comprising outer layers of a copolymer of ethylene and vinyl acetate and a gas-barrier layer of a VDC, and the thus disclosed film has solved the problem, however, since the rigidity of the thus disclosed film is somewhat insufficient, the laminate film cannot exhibit a satisfactory operational processability in packaging foodstuffs.
For improving the rigidity of the laminate film, an utilization of a layer of an ionomer has been proposed. For instance, U.S. Pat. No. 4,226,822 discloses a laminate film of a total thickness of 35 to 90 .mu.m, which comprises at least one of the outer layers of an ionomer of 20 to 50 .mu.m in thickness, two adhesive layers and a gas-barrier layer of a VDC.
Although the thus disclosed laminate film shows an adequate rigidity and accordingly, exhibits a satisfactory operational processability in packaging foodstuff, in the case where a laminate film is prepared in a tubular form while having the ionomer layer as the outer layer of the tubular film, because of the hydrophilicity of the ionomer, the outer layer of the ionomer is whitened by water attached in the quenching step. The thus whitened outer layer deteriorates the appearance of the film itself and also the packaged foodstuffs. Also in the case where the innermost layer of the tubular form is the ionomer, the ionomer layer to be sealed when the foodstuff is packaged is required to have a certain extent thickness. As a result, such a heat-shrinkable laminate film tends to become rigid and to lose flexibility and accordingly the heat-shrinkable laminate film is not suitable for a large amount of foodstuff. These demerits remain as far as the ionomer layer is used as the outer layer or inner layer of the tubular form of the laminate film.
In consideration of the merits and demerits of the use of an ionomer layer, the present inventors have tried to prepare a heat-shrinkable laminate film having at least one ionomer layer not as the outer layer (or the inner layer) but as the intermediate layer, and have attained to the present invention.
Namely, the heat-shrinkable laminate film according to the present invention comprises outer layers of flat sheet form or the outer layer and the inner layer of tubular form of a polyolefin, a gas-barrier layer of a VDC, at least one intermediate layer of an ionomer and adhesive layers disposed between any two of the layers, the total thickness of the intermediate layer(s) of the ionomer being 5 to 20% of the total thickness of the heat-shrinkable laminate film.
Because of the presence of at least one ionomer layer of a specified thickness between the outer layer of a polyolefin and the gas-barrier layer of a VDC, the heat-shrinkable laminate film according to the present invention shows an adequate rigidity, thereby overcoming the difficulty due to elongation of the film by the weight of the article to be packaged in packaging operation, and a more uniform stretchability than that of the film disclosed in Japanese Patent Application Laying-Open No. 58-128821 (1983). The laminate film according to the present invention is heat-shrinkable, impermeable to gaseous oxygen and free from whitening.